An integrated circuit (IC) is a microelectronic device that houses multiple electronic circuits on a chip. ICs are built by lithographic fabrication of numerous transistors on a silicon chip. Similarly a photonic IC (PIC) is a device that houses integrated photonic functions on a chip. A PIC fabricated from nanomaterials and/or via a nanolithography process is called a nanophotonic integrated circuit. Although the physics of photons (neutral particles) and electrons or holes (charge carrying particles) are different from each other, the evolution of the electronic IC provides an analogy to PICs. The development of PICs promises to increase functionality, density, and significantly reduced cost when compared to optical components that are assembled from discrete photonic elements. While there are several existing methods for designing and building fiberoptic networks and systems, the fundamental breakthrough required to meet dramatically reduced cost points is the introduction of viable PIC technology that must meet a few important criteria such as:
(i) PIC technology must be capable of creating a broad range of optical functions out of a single fabrication process.
(ii) The means must exist for PICs to be readily manufactured at low cost in high volume.
(iii) The capability must be developed to aggregate individual optical functions into more complex arrangements within the PIC technology and with other optical technologies.
While classical electronic computing has made significant advances to the modern generation of fast computers and the Internet, there are currently applications needs that require even faster speed, higher bandwidth, more robust deployment capability and cheaper methods of fabrication. In the field of photonics, efforts to achieve a true integrated platform for fiberoptic communication and computing that can parallel the electronic counterpart have been unsuccessful. Many of the optical components available today, whether passive, active, or hybrid combinations of the both, are analogous to the vacuum tubes that predate the modern transistors and ICs. This is primarily because silicon, as an optical material, poses several challenges that make it almost impossible to produce a true PIC that can replicate the IC revolution. These issues are fundamental limitations that arise from material properties, and result in difficulty of externally controlling.
Silicon structures are difficult to control externally for optical switching. Poor light emission from silicon devices is attributed to silicon's indirect band gap. It is difficult to obtain optical amplification with silicon because of difficulty in doping silicon with amplifying ions. It is also difficult to fabricate modulators because of silicon's low electro-optic coefficient. Therefore, there is an expressed need to develop novel devices that can overcome the limitations of silicon-based photonics. Novel materials and novel methods are necessary for enhancing the interaction of light with matter. Devices with novel functionalities for guiding light, switching, amplifying, and modulating need to be developed. In addition, a long awaited need is the ability to combine electronics with photonics by fabricating PICs on a silicon wafer. Such devices will form the basis for an ultimate photonic integrated circuit. If a smart nanomaterial is used to build such a PIC, it is termed as an nPIC. In the field of photonics, a smart material is defined as a material that can provide multiple functionalities such as waveguiding, amplifying, and modulating of photonic signals
For a truly integrated photonic technology, a smart material system is necessary that functions in a similar fashion for photonics, as silicon does for electronic IC. At the fundamental level, electronic IC requires only a periodic arrangement of numerous “transistors” (or p-n junctions) in a smaller dimension. Because IC functionality depends primarily on the movement of charge carriers, the features necessary to carry out these functionalities can be as small as lithographically possible, the latest being a 90 nm line width.
Contrasting this with photonics, features suitable for photon guiding and coupling must first be able to accommodate the photons. As a result the waveguides must at least be of the order of the wavelength of the photon. This requirement is, in some aspects, an advantage for photonics because lithographic processes are readily available. On the other hand there exists a new set of challenges to overcome because of the difficulties with silicon material properties explained above, and because of the fact that silicon cannot do everything necessary for photon manipulations. Hence, the need for a new material-based technology.
From a fundamental point of view the basic functionalities necessary to build a chip that can ultimately be termed as an “optical processor” include the following: light guiding, light amplifying, light modulating, signal processing, attenuating, sensing, on-chip light source and detector, on-chip interconnect, and inputting and outputting of the signal. Being able to build all of these functionalities on a single chip that would represent a true PIC is an ambitious goal. Nevertheless, this is what one must achieve to make a breakthrough, not only from a scientific and technology point of view, but also to be able to solve the problems that currently exist.
The term “photonic integrated circuit” has been assigned to several different kinds of chip-based devices. For example, U.S. Pat. No. 5,863,809 (Jan. 26, 1999) titled “Manufacture Of Planar Photonic Integrated Circuits,” describes integration of a laser (active) waveguide with a passive waveguide in a coplanar configuration. As another example, U.S. Pat. No. 5,703,974 (Dec. 30, 1997) titled “Semiconductor Photonic Integrated Circuit And Fabrication Process Therefor,” describes InP based photonic integrated circuit wherein an active region and a passive region are coupled. Other examples of using the term photonic integrated circuit in one form or other can be found in US Pat. App. No. 2004/0067006 A1 (Apr. 08, 2004) titled “Transmitter photonic integrated circuit (TXPIC) chips,” in US Pat. App. No. 2003/0099425 A1 (May 29, 2003) titled “Optical Communication Module With One or More Photonic Integrated Circuit (PIC) Chips and an External Booster Optical Amplifier for Photonic Integrated Circuits,” in U.S. Pat. No. 6,224,667 (May 01, 2001) titled “Method for Fabricating Semiconductor Light Integrated Circuit,” and in US Pat. App. No. 2004/0105476 A1 (Jun. 03, 2004) titled “Planar Waveguide Surface Emitting Laser and Photonic Integrated Circuit.”
In general, the term PIC has been used for any photonic device that accommodated optical functionality on a chip. It is apparent that, while the term PIC has been used for a number of years, devices based on those PICs are not in wide use in practical network equipment. The primary reason is the typical unfavorable cost and performance issues of the available PIC devices that need to be improved. Also the above referenced PICs lack features to be able to term them as a complete PIC or an optical processor.